1. Field of the Invention
The present invention relates to the field of resistor-based memory circuits. More particularly, it relates to a method for precisely sensing the resistance value of a resistor-based memory cell, for example, an MRAM magnetic memory cell.
2. Description of the Related Art
FIG. 1 shows one example of a resistor based memory. The memory includes a memory cell array 90 having a plurality of row lines 100 arranged in normal orientation to a plurality of column lines 110. Each row line is connected to each of the column lines by a respective of resistor 120.
A magnetic random access memory (MRAM) is one approach to implementing a resistor based memory. In an MRAM, each resistive memory cell includes a magnetizable film. The resistance of the cell varies, depending on the magnetization state of the film. Logical data can be stored by magnetizing the film of particular cells so as to represent the logic states of the data. The stored data can be read by measuring the resistance of the cells, and interpreting the resistance values measured as logic states. Making the required resistance measurements, however, is problematic.
In a resistance memory, one resistance value, e.g., a higher value, may be used to signify a logic xe2x80x9cHIGHxe2x80x9d while another resistance value, e.g., a lower value, may be used to signify a logic xe2x80x9cLOW.xe2x80x9d The stored logic state can be detected by measuring the memory cell resistance using Ohm""s law. For example, resistance is determined by holding voltage constant across a resistor and measuring, directly or indirectly, the current that flows through the resistor. Note that, for MRAM sensing purposes, the absolute magnitude of resistance need not be known; only whether the resistance is above or below a value that is intermediate to the logic high and logic low values.
Sensing the logic state of an MRAM memory element is difficult because the technology of the MRAM device imposes multiple constraints. In a typical MRAM device an element in a high resistance state has a resistance of about 1 Mxcexa9. An element in a low resistance state has a resistance of about 950 Kxcexa9. The differential resistance between a logic one and a logic zero is thus about 50 Kxcexa9, or 5% of scale.
Accordingly, there is a need for a simplified resistance measuring circuit able to repeatably and rapidly distinguish resistance values varying by less than 5% on a one Mxcexa9 scale.
The invention provides a method and apparatus for measuring the resistance of a resistive memory element. The resistance is measured by charging a capacitor, allowing the capacitor to discharge through a selected resistive memory element while maintaining a substantially constant voltage across the resistive memory element, sensing the charge remaining on the capacitor, repeatedly recharging the capacitor with a pulse of definite charge each time the capacitor voltage drops to a predetermined value, and determining a time average current into the capacitor based on a duty cycle of the recharging pulses. Knowledge of the time average current into the capacitor, yields the current flowing into the resistor since the current flowing into the capacitor must equal the current flowing out of the capacitor and into the resistor. One can measure or set the voltage across the resistive memory element and determine the resistance of the element from the current through the element and the voltage across it.
In various aspects of the invention, the actual resistance of the memory element is not calculated. Instead, the number of capacitor charging pulses is counted, and the numerical count thus acquired is compared to a reference count value. The reference value is chosen to lie between count values representing logical one and logical zero. Therefore a count value greater than the reference indicates one logical state, and a count value less than the reference value indicates another. In a further aspect of the invention, more than one reference value is established, and a memory element capable of exhibiting more than two resistance values is used. Consequently the memory element may store more than two logical values. The logical values are determined based on the relationship between the count value counted and the standard values used to establish thresholds between logical values.
In a further aspect, the apparatus and method of the invention may be used to measure the resistance or impedance of any resistive or impedance device.
These and other aspects and features of the invention will be more clearly understood from the following detailed description which is provided in conjunction with the accompanying drawings.